Medium voltage drives comprise power/frequency converters, which are used in various sectors to drive a variety of machines and processes.
Basic features of an electrical converter are power flow and energy storage. For example, the electrical converter may absorb power from a three-phase AC power source (such as an electrical grid), storing this power as energy in DC form using capacitors or inductors, and finally converting this stored energy back to AC form and driving an electric machine, such as an electrical motor. This power flow also may be reversed, for example, when wind energy is harvested, i.e. a wind turbine may convert mechanical power to electric power, which is rectified and stored. Finally, the stored DC energy may be inverted and fed back to the grid in an AC form. Furthermore, alternating the power flow at least at the rectifier side and/or the inverter side is also possible for short periods of time to allow extra controllability of the medium voltage drive.
The control of the power flow may be based on an estimation of a fundamental component of the AC voltages at the connection point of the medium voltage drive to the grid. For example, in US 2014/0043014 A1, measurement signals of AC voltages are filtered in a complex way to estimate fundamental components of the AC voltages.
“Frequency Adaptive Least-Squares Kalman Technique for Real-Time Voltage Envelope and Flicker Estimation”, IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, IEEE SERVICE CENTER, PISCATAWAY, N.J., USA, vol. 59, no. 8, 1 Aug. 2012 provides a method for estimating a fundamental component of an AC voltage. Also in US 2012/0253742 A1 a method for estimating a fundamental component of an AC voltage is disclosed.
In EP 2 623 997 A1 a method and an apparatus for determining a phase and a frequency of an electric quantity being associated with an operation of an electrical device are described.